A user's heart rate is a physiological characteristic that may be used to determine and track one or more performance characteristics of the user. A photoplethysmogram (PPG) signal provides information that can be used to determine physiological metrics of an individual, such as a heartbeat signal and other cardiac information. Devices that generate a PPG signal using optical techniques typically include an emitter configured to output light toward skin of the user and a receiver, which may include a photodiode and electrical circuitry, configured to generate a light intensity signal based on a light reflections received from the photodiode and using the electrical circuitry to generate a PPG signal based on the light intensity signal. The PPG signal includes a cardiac component, a motion component and a respiratory component. The signal-to-noise ratio (SNR) for the desired cardiac component of a PPG signal is sensitive to movements of the user and/or a device configured to generate the PPG signal that is worn by the user. Even slight movements of the user or the device may rapidly degrade the SNR for the cardiac component of the PPG signal. Because the PPG signal is sensitive to such noise, the individual is typically still or motionless when physiological metrics are monitored.
The exemplary PPG signals illustrated in FIGS. 1 to 3 include artifacts from other interfering components generally attributed to a source of interference, such as movement of the device. For example, FIG. 1 depicts a PPG signal generated by a receiver based on received intensities of received light reflections from an individual engaged in walking (period from 0 to 150 seconds), running (period from 150 seconds to 1500 seconds) and jogging (period from 1500 seconds to 3000 seconds). The PPG signal of FIG. 1 depicts a dominant motion component associated with a cadence of the user in each activity type. Similarly, FIG. 2 depicts a PPG signal generated by a receiver based on received intensities of received light reflections from an individual engaged in a few strides of jogging over a duration of eight seconds. The PPG signal of FIG. 2 depicts a dominant motion component associated with a cadence of the user while jogging. FIG. 3 depicts a graph of a slightly drifting or undulating PPG signal, which includes a PPG signal generated by a receiver based on received intensities of received light reflections from an individual engaged in sedentary or comparatively idle activity over a period of approximately nine seconds. As can be readily observed from the PPG signals of FIGS. 1-3, the PPG signal of FIG. 3 is more stable (i.e., contains fewer motion components resulting from physical activity) than the PPG signals of FIGS. 2 and 3, which depict PPG signals associated with users engaged in more strenuous activity and thus relatively more motion components resulting from the user movement.
Unlike a medical environment in which a user may be sedentary (i.e., a patient receiving medical care receives attention while resting in a bed), typical daily activities, which may include moving, walking, running, swimming, driving, etc., may introduce interference effects on the PPG signal during such activities. It is desirable to minimize such interference effects to better determine the desired physiological aspect of the individual.